1. Field of the Invention
The present invention relates to a wide-angle lens system for a single lens reflex camera.
2. Description of the Related Art
In a single lens reflex camera, the lens system thereof needs to have a back focal distance which is long enough to provide a quick-return mirror between the photographing lens system and the image plane. Accordingly, a retrofocus-type lens system in which the back focal distance is longer than the focal length of the lens system has been generally employed. Due to the optical arrangement of the retrofocus-type lens system, in which a negative first lens group and a positive second lens group are provided, in this order from the object, an optical power (hereinafter, power) distribution over the first and second lens groups is asymmetrical with respect to the diaphragm. As a result, negative distortion and field curvature tend to increase to the extent that an increase of optical performance of the lens system is hindered. Particularly, in the case of a wide-angle lens system having a smaller F-number, i.e., a faster-lens system, coma occurred on the sagittal plane (hereinafter, sagittal coma) and higher-order aberrations increase.
It is an object of the present invention to provide a wide-angle lens system having a small F-number (preferably, smaller than 2), thereby distortion occurred in the negative direction, field curvature, and sagittal coma can be sufficiently corrected.
In order to achieve the above-mentioned object, there is provided a wide-angle lens system including a negative first lens group and a positive second lens group, in this order form the object. The first lens group includes a positive lens element, and at least one negative lens element, in this order form the object. The wide-angle lens system satisfies the following conditions:
0 less than (xcfx861-2+xcfx862-1)/xcfx86 less than 0.3xe2x80x83xe2x80x83(1)
0.5 less than HF/HL less than 0.9xe2x80x83xe2x80x83(2)
wherein
xcfx861-2=(1xe2x88x92n1)/r1-2;
xcfx862-1=(n2xe2x88x921)/r2-1;
xcfx86=1/f;
n1 designates the refractive index of the positive lens element of the first lens group;
n2 designates the refractive index of the most object-side negative lens element of the first lens group;
r1-2 designates the radius of curvature of the image-side surface of the positive lens element of the first lens group;
r2-1 designates the radius of curvature of the object-side surface of the most object-side negative lens element of the first lens group;
f designates the focal length of the entire lens system;
HF designates the height from the optical axis of a bundle of axial light rays with respect to the most object-side surface of the first lens group; and
HL designates the height from the optical axis of a bundle of axial light rays with respect to the most object-side surface of the second lens group.
The wide-angle lens system preferably satisfies the following condition:
0 less than d1/f less than 0.2xe2x80x83xe2x80x83(3)
wherein
d1 designates the axial distance, in the first lens group, between the positive lens element and the most object-side negative lens element.
The second lens group can include a positive first sub-lens group 2F and a positive second sub-lens group 2R, in this order from the object. The second lens group preferably satisfies the following condition:
0.1 less than d2f less than 0.5xe2x80x83xe2x80x83(4)
wherein
d2 designates the axial distance between the first sub-lens group 2F and the second sub-lens group 2R, when an object at an infinite photographing distance is in an in-focus state.
The second lens group preferably includes at least one aspherical surface thereon, and the aspherical surface satisfies the following condition:
xe2x88x920.2 less than xcex94V less than 0xe2x80x83xe2x80x83(5)
wherein
xcex94V designates an aspherical-surface term in a distortion coefficient.
In a focusing operation, from infinity toward an object at a closer distance, performed by the wide-angle lens system of the present invention, a first focusing lens group Fa in which the first lens group and the first sub-lens group 2F are arranged to be integrally moved, and a second focusing lens group Fb constituted by the second sub-lens group 2R are preferably arranged to move independently from the image side toward the object side so that the distance therebetween becomes shorter. More concretely, the first focusing lens group Fa, and the second focusing lens group Fb can be moved according to at least the following patterns:
(i) the first focusing lens group Fa and the second focusing lens group Fb are moved from the image side toward the object side at a substantially constant ratio of the traveling distance of the second focusing lens group Fb to that of the first focusing lens group Fa; or
(ii) the first focusing lens group Fa and the second focusing lens group Fb are moved from the image side toward the object side in such a way that the shorter the object distance is, the larger a ratio of the traveling distance of the second focusing lens group Fb to that of the first lens group Fa becomes.
The positive second sub-lens group 2R can include a positive first minor sub-lens group 2R-1, a negative second minor sub-lens group 2R-2 and a positive third minor sub-lens group 2R-3. In this optical arrangement, a positive lens element in the positive first minor sub-lens group 2R-1 preferably satisfies the following condition:
xe2x80x831.7 less than N2R-1pxe2x80x83xe2x80x83(6)
wherein
N2R-1p designates the refractive index of the positive lens element of the positive first minor sub-lens group 2R-1.
Similarly, a negative lens element in the negative second minor sub-lens group 2R-2 preferably satisfies the following condition:
1.7 less than N2R-2nxe2x80x83xe2x80x83(7)
wherein
N2R-2n designates the refractive index of the negative lens element of the negative second minor sub-lens group 2R-2.
Still further, a positive lens element in the positive third minor sub-lens group 2R-3 preferably satisfies the following condition:
1.7 less than N2R-3pxe2x80x83xe2x80x83(8)
wherein
N2R-3p designates the refractive index of the positive lens element of the positive third minor sub-lens group 2R-3.
The above wide-angle lens system according to the present invention is suitable for a lens system with a F-number of less than 2, i.e., a faster lens system. In addition, the wide-angle lens system according to the present invention is suitable for a lens system with the half-angle-of-view of more than 33xc2x0.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-224765 (filed on Jul. 26, 2000) which is expressly incorporated herein by reference in its entirety.